The present invention relates to a proton-conducting electrolyte membrane. In particular the present invention relates to a proton-conducting electrolyte membrane having a high conductivity, high mechanical and chemical stability, high flexibility and thermal stability which can be used in fuel cells over a wide temperature range. Furthermore, the present invention relates to a method of producing the proton-conducting electrolyte membrane and the use of at least one proton-conducting electrolyte membrane in the form of a membrane electrode assembly (MEA) in a fuel cell.
For a stationary and mobile electric power generation, e.g., for road vehicles, space travel, power plants, etc., fuel cells are being investigated and developed as a possible electric current source to an increasing extent. A fuel cell is a galvanic element in which chemical energy is converted directly into electric energy, i.e., not by way of thermal energy. A single cell consists of two invariant electrodes between which there is an invariant electrolyte. The fuel cell continuously supplies electric current by continuously supplying the substance that is to be oxidized, i.e., the so-called fuel, e.g., hydrogen, which is obtained by cleavage of natural gas, methanol, hydrazine, ammonia, etc., and the oxidizing agent, e.g., oxygen, and by continuously removing the oxidation products such as water.
With the beginning of use of polymer membranes as the invariant solid electrolyte, attention has been directed at proton-conducting membranes based on ionomers containing perfluorinated sulfonic acid units, e.g., perfluoroalkyl sulfonic acid polymer electrolytes. Such a membrane is available under the brand name Nafion® from DuPont, for example. Such membranes are heterogeneous systems. The hydrophilic and hydrophobic polymer building blocks form a cluster structure into which water is incorporated as a prerequisite for a high conductivity. With these heterogeneous systems, charge transport is bound to the liquid, i.e., aqueous phase. The fact that water molecules function as proton carriers in these proton conductors means that the water concentration in the cell must be kept constant. The latter is problematical because water is also produced as an oxidation product in the reaction and therefore must be removed in a controlled manner. These fuel cells also operate only at temperatures below 100° C., likewise due to the role of the water molecules as proton carriers, because at temperatures above 100° C. water is expelled from the membrane. This is associated with a decline in conductivity, which is why these membranes are limited to use in a temperature range below 100° C. (O. Savadogo et al., Journal of New Materials for Electrochemical Systems 1 (1998), pp. 47-66). Another problem with membrane fuel cells based on Nafion is that the catalysts used are particularly sensitive to catalyst poisons such as carbon monoxide in the temperature range below 100° C. and in general are less effective there.
U.S. Pat. No. 5,525,436 describes a solid polymer electrolyte membrane which includes a proton-conducting polymer that is thermally stable at temperatures up to 400° C. The proton-conducting polymer may be a basic polymer, preferably polybenzimidazole (PBI) doped with a strong acid, preferably sulfuric acid or phosphoric acid. When a basic polymer such as PBI is doped with a strong or stable acid such as sulfuric acid or phosphoric acid, it results in a polymer electrolyte which forms a single-phase system in which the acid is complexed by the polymer in contrast with the heterogeneous water-based systems described above in which the charge transport is bound to the liquid phase.
In the single-phase electrolyte systems described in U.S. Pat. No. 5,525,436 (also in contrast with the heterogeneous systems), commercially relevant conductivities are achieved only at temperatures above 100° C. The conductivity of the membranes depends on the phosphoric acid concentration and the doping time but does not have any mentionable dependence on the water content. The removal of acid at temperatures above 100° C. is negligible. For these reasons and because of the high oxidation stability of the polymer, PBI membranes doped with phosphoric acid as the electrolyte and separator have been developed for fuel cells in the working temperature range above 100° C., e.g., approximately 160° C.
The phosphoric acid-doped PBI membranes are produced in a two-step process with the PBI being dissolved in dimethylacetamide (DMAC) containing 2% lithium chloride (LiCl) in the first step and the solution then being converted to the form of a film by casting or spraying. After drying the film, the LiCl is extracted from the film with water, yielding an amorphous and flexible membrane as an intermediate product. In the second step, the membrane is doped with sulfuric acid (H2SO4) and/or preferably with phosphoric acid (H3PO4). The doping converts the membrane to a partially crystalline form. Therefore and in combination with the degree of doping, the mechanical load-bearing capacity of the membrane is reduced. This effect is increased by additional swelling with water, e.g., in storage of the membrane or due to uptake of water which occurs during operation of the fuel cell.
To increase the mechanical strength, it has been proposed, e.g., in WO 00/44816 that the polymer membrane of PBI, for example, be crosslinked with the help of a crosslinking agent. Although this makes it possible to increase the mechanical strength and thus partially eliminate the problem of brittleness, such crosslinked polymer electrolytes have a reduced conductivity and swellability with phosphoric acid.
The advantage of being able to use the single-phase phosphoric acid-doped PBI membranes described above at temperatures above 100° C. must be seen against a number of disadvantages. These membranes can only be produced in a two-step process, which is time consuming and also wasteful of materials and thus increases production costs. The polymer electrolyte membrane has a declining mechanical strength because of the crystallinity that is already present at the beginning and increases further and because of the high degree of doping with the dopant and this reduced mechanical strength can lead to deformation of the membrane or even its destruction. For this reason, the membrane must be stored in the absence of water before use. Furthermore, the power of a fuel cell containing such polymer electrolyte membranes and operated at a temperature below 100° C. declines over time because the membrane takes up water in this temperature range, which dilutes the acid (e.g., phosphoric acid) and washes it out.
To simplify the two-step production process, U.S. Pat. No. 5,716,727 describes a process for producing PBI membranes doped with phosphoric acid. A solution consisting of PBI, phosphoric acid and trifluoroacetic acid as the solvent is prepared and processed to yield a membrane by casting and evaporating the solvent.
Although this method makes it possible to produce a phosphoric acid-doped PBI membrane in a single step, this method also has some serious disadvantages. First, the resulting membrane also has an initial crystallinity which increases further and therewith brittleness with all the disadvantages described above. A significant disadvantage of this method, however, is the necessity of using trifluoroacetic acid. Trifluoroacetic acid is highly volatile, extremely aggressive and toxic and has a low flash point. Therefore this process can be carried out only in closed systems using stringent safety measures. At any rate, the proposed method causes a great deal of environmental pollution, however, because of the trifluoroacetic acid which is difficult to dispose of.